F. Wenger

Electrochemical techniques for studying tribocorrosion processes

The combined corrosion-wear degradation of materials in sliding contacts immersed in electrically conductive solutions can be investigated in situ by electrochemical techniques. Such techniques are the open circuit potential measurements, the potentiodynamic polarization measurements, and the electrochemical impedance measurements. In this paper, capabilities and present limitations of these techniques are discussed based on a tribocorrosion study of a AISI 316 stainless steel and an iron-nickel alloy immersed in aerated 0.5 M sulfuric acid and sliding against a corundum counterpart. Some novel insights into the tribocorrosion mechanism gathered in this way are discussed.

Composite Electrodeposition to Obtain Nanostructured Coatings

Nanocomposite coatings are obtained by electrochemical deposition of silicon carbide nanoparticles (mean diameter 20 nm) in a nickel plating bath. The influence of nanoparticles on nickel electroplating is studied by electrochemical impedance spectroscopy at cathodic potential, and cathodic polarization diagrams are performed in the electrolyte with and without nanoparticles. We show that incorporation of 20 nm SIC particles into nickel deposits modifies the impedance and cathodic polarization diagrams in correlation with the modification of the surface morphology and crystallinity of the nickel matrix

Tribocorrosion behaviour of anodic treated titanium surfaces intended for dental implants

Tribocorrosion plays an important role in the lifetime of metallic implants. Once implanted, biomaterials are subjected to micro-movements in aggressive biological fluids. Titanium is widely used as an implant material because it spontaneously forms a compact and protective nanometric thick oxide layer, mainly TiO2, in ambient air. That layer provides good corrosion resistance, and very low toxicity, but its low wear resistance is a concern. In this work, an anodizing treatment was performed on commercial pure titanium to form a homogeneous thick oxide surface layer in order to provide bioactivity and improve the biological, chemical and mechanical properties. Anodizing was performed in an electrolyte containing β-glycerophosphate and calcium acetate. The influence of the calcium acetate content on the tribocorrosion behaviour of the anodized material was studied. The concentration of calcium acetate in the electrolyte was found to largely affect the crystallographic structure of the resulting oxide layer. Better tribocorrosion behaviour was noticed on increasing the calcium acetate concentration. (Some figures may appear in colour only in the online journal)

An electrochemical noise study of tribocorrosion processes of AISI 304 L in Cl− and media

Electrochemical noise measurements were performed to investigate the intrinsically stochastic character of the tribocorrosion process. Unidirectional sliding tests (pin-on-disc) were performed using AISI 304L stainless steel sliding against corundum. Experiments were carried out in Cl− and containing media under open-circuit and potentiostatic polarization conditions. The power spectral density (PSD) of the current and potential signals showed a strong dependence on the sliding frequency but did not depend significantly on the normal load between 5 and 20 N. The fluctuations of the tangential and normal loads were also recorded, and a critical comparison between the PSD of the electrochemical response and the PSD of the mechanical solicitation (load) is proposed. At high frequencies (f > 0.1 Hz), the PSD of current or potential fluctuations have significantly different shapes than the PSD of load signals: the electrochemical signal PSD is governed by the dynamic balance between local depassivation and repassivation which only depends on the kinetics of the electrochemical phenomena. For lower frequencies, a plateau is observed for both the electrochemical PSD and the load PSD. The electrochemical signal is then governed by the continuous depassivation induced by sliding which appears as a low frequency component. These results suggest that the electrochemical noise technique investigated in the frequency domain might be a promising electrochemical tool for successfully unfolding tribocorrosion signatures for material parings in sliding-corrosion tests.

Local Electrochemical Studies and Surface Analysis on Worn Surfaces

Specimens of duplex stainless steel UNS S32304 immersed in distilled water were worn under sliding friction conditions. The electrochemical microcell technique and local Auger spectroscopy were then applied to these heterogeneous surfaces to determine the influence of surface chemical composition and surface defects on the electrochemical behavior of each phase of the stainless steel in and out of the wear track. Experiments were performed both in 1 M NaCl, pH 3 (where the passive film is stable), and 0.5 M HCl (where an active peak is observed). In addition, the scanning vibrating electrode technique was combined with a modified microcell to study microgalvanic processes between the surrounding passive surface and the track at the OCP value.

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. III. Influence of hydrogen absorption for stressed electrodes

In this third part of the study of the electrochemical noise generated by hydrogen evolution on cathodically-polarised AISI 4140 steel specimens in 0.5 M sulphuric acid, a tensile stress was applied to the specimens. The two components of the measured potential fluctuations DeltaV, namely ohmic-drop fluctuations DeltaR(e)I and faradaic potential fluctuations DeltaE, were analysed. After bubble size homogenisation, the effect of an applied stress was an increase in the level of the power spectral densities (PSD) Psi(V) and Psi(E) of the Delta V and DeltaE fluctuations, indicating a change in metal-hydrogen-interactions related to damages in the metal induced by the combined actions of stress and hydrogen embrittlement. In the meanwhile, the PSD Psi(ReI) of the ohmic-drop fluctuations did not exhibit any change, revealing that the departure rate and size of hydrogen bubbles were not modified by the internal damages in the specimen. The time evolution Of Psi(E) up to fracture could be explained by the enhancement of hydrogen penetration into the metal induced by the increase in the density of microdefects or crack advances inside the metal.

Fluctuations of Permeation Rate Through an Iron Membrane Induced by Hydrogen Bubbles

Using the electrochemical permeation technique, the authors have recorded fluctuations of the concentration of absorbed hydrogen in an Armco iron membrane during cathodic hydrogen charging in acidic solution. The fluctuations coincide quite closely with the dynamics of hydrogen bubbles at the cathode surface, which were followed both visually and by recording cathode-potential and ohmic-resistance fluctuations. The local flux of hydrogen into the iron membrane gradually decreased when there is an attached bubble growing and quickly increases when the bubble departs. An excellent correlation was found between the fluctuations of the concentration of absorbed hydrogen in the first layers of metal and the fluctuations of the cathode potential.

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. II. Identification of potential fluctuation sources for unstressed electrodes

In this second part of the study of electrochemical noise (EN) generated by hydrogen evolution on a vertical cylindrical AISI 4140 steel electrode under galvanostatic control in 0.5 M sulphuric acid, the potential fluctuations induced by the growth and detachment of hydrogen bubbles at the electrode surface were analysed. They could be related to fluctuations of various quantities: electrode active surface due to bubble screening effects, concentration of dissolved hydrogen in the electrolyte close to the electrode surface, and metal-hydrogen interactions (MHI) on or beneath the electrode surface. The existence of MHI and their influence on faradaic potential fluctuations could be revealed by comparing the noise features on steel and platinum. The influence on EN of the charging cathodic current density, the presence of dissolved oxygen in the solution, and the electrode rotation speed was investigated in the absence of stress applied to the electrode. In the third paper of this series, the effect of hydrogen embrittlement on potential fluctuations of stressed electrodes will be examined.

Study of the Anodic Current‐Voltage Curve of an Iron‐Nickel Alloy in Normal Sulfuric Acid

The anodic steady-state polarization curve of an Fe-31% Ni rotating disk electrode in normal sulfuric acid was studied. As in the case of iron in similar conditions, a Z-shaped curve was found and, in a part of the curve, called the transition region, several electrochemical processes (homogeneous dissolution, intergranular dissolution, passivation) coexist on different areas of the electrode surface. The experimental study of the current density distributions and dissolution morphologies on the electrode surface, in the different regions of the curve, allowed one to test the validity of a theoretical model developed to explain the curves of iron electrodes. In this way, the authors could point out significant differences between the processes occurring at iron and iron-nickel electrodes, and they obtained the current density-potential relationship describing the kinetic laws of the processes. This work is the first and essential step of a research on the influence of the nickel content on the dissolution and passivation mechanisms of the iron-nickel alloys.

Testing protocol for the study of bio-tribocorrosion

Abstract: Articulating systems are complex mechanical systems operating under sliding, rotation, slip, vibrations, weight bearing, and loading conditions. A problem in mechanical systems is the occurrence of tribocorrosion, involving a synergism between corrosion and mechanical wear, and causing increased material degradation. A tribocorrosion protocol allowing separation of the mechanical, corrosive and synergetic contributions to the total loss of material is described, based on electrochemical measurements performed before and during unidirectional sliding. As a case study, the protocol is implemented to test the tribocorrosion resistance of CoCrMo alloys under sliding. The different components of the total wear related to corrosive and mechanical contributions are calculated.